Oxidation catalyst comprising variable density activator



3,377,269 OXIDATION CATALYST COMPRISTNG VARIABLE DENSITY ACTIVATORHerman S. Bloch, Skokie, lll., assignor to Universal Oil ProductsCompany, Des Pl'aines, 111., a corporation of Delaware No Drawing.Continuation-impart of application Ser. No. 369,279, May 21, 1964, whichis a continuation-in-part of application Ser. No. 75,666, Dec. 14, 1960.This application June 15, 1964, Ser. No. 375,354

13 Claims. (Cl. 208466) ABSTRACT OF THE DISCLOSURE A catalyst particlecomprising Pt on high surface area alumina, a minor portion of saidsurface area comprising a number of small localized spots of Pt at ahigh density of the order of 20,000 micrograms of Pt/m. and theremaining surface area thereof having a uniform distribution of Ptthereon at a low density of the order of 09-10 micrograms of Pt/m. thetotal Pt content of said particle being 0.05%-0.2% by weight.

This application is a continuation-in-part of my copending applicationSer. No. 369,279, filed May 21, 1964, which in turn is acontinuation-in-part of my copending application Ser. No. 75,666, filedDec. 14, 1960, now abandoned.

This invention relates to a novel oxidation catalyst and to itspreparation and use. More particularly the invention is directed to animproved oxidation catalyst comprising a variable density activatoruseful for converting exhaust gas streams, such as those emanating fromvehicular and stationary internal combustion engines, to less harmfulproducts.

The desirability and importance of removing certain constituents fromvehicular exhaust gases is recognized. The generally unavoidablyincomplete combustion of hydrocarbon fuels by the internal combustionengine results in the generation of substantial quantities of unburnedhydrocarbons and other undesirable products which are released to theatmosphere through the exhaust line. With the ever increasing number ofautomobiles, particularly in urban areas, the discharge of deleteriousmatter into the atmosphere may reach significant proportions. Theundesirable combustion products include, for example, unsaturatedhydrocarbons, partially oxidized hydrocarbons such as alcohols, ketones,aldehydes and acids, carbon monoxide, and various oxides of nitrogen andsulfur. Some of these undesirable products are believed to react withatmospheric oxygen, under the influence of sunlight, to produce what isnow commonly referred to as smog.

The discharge of exhaust gases from automotive engines is only onesource of atmospheric pollution. Although described with particularreference to the conversion of such exhaust gases, the present inventionis equally well suitable for use with diesel engines, butane engines,natural gas engines and the like. Other sources of atmospheric pollutioninclude waste material from stationary units such as large internalcombustion engines for driving pumps, compressors and generators; fluegas power recovery units; exhaust fumes from various industrialoperations such as the printing industry, the tanning industry andvarious chemical industries. For example, in the printing industry,inks, dyes and the like contain hydrocarbons and other chemicalcompounds which, either in the same or modified form, accumulate withinthe surroundings and are vented to the atmosphere by blowers or fans. Inthe chemical field, for example, the manufacture of phthalic anhydrideby the oxidation of naphtha- 3,377,269 Patented Apr. 9, 1968 gases bycatalytic means prior to their discharge into the atmosphere, theobjective being to convert at least a substantial portion of theunburned or incompletely burned hydrocarbons and carbon monoxide intocarbon dioxide and water.

With regard to automotive and other vehicular applications, the catalystis usually disposed as a fixed particleform bed placed in a suitablecontainer or catalytic convertor which is installed in the engineexhaust gas line. The catalytic convertor may be of the through-flow,crossflow or radial-flow design and may supplant or be combined with thenormal acoustic mufller. Secondary or combustion air is injected aheadof the convertor inlet usually 'by means of an aspirator or by asuitable enginedriven compressor. The rate of secondary air flow isusually adjusted or maintained to provide from about 10% to about 30%excess air to insure reasonably high conversion levels under allconditions of driving.

An important consideration in the formulation of an exaust gas oxidationcatalyst for vehicular use is to attain a relatively low ignitiontemperature, or threshold activation temperature, so that the conversionreactions are self-sustaining within a minimum time after startup fromcold engine conditions and the emission of unconverted gases isaccordingly held to a minimum. Depending on the makeup of exhaust gasesand at specified conditions of combustible content and percent oxygen,all catalytic compositions are characterized by such ignitiontemperature, below which virtually no conversion of the exhaust gasestakes place. After the catalyst bed has been brought up to operatingtemperature, however, the exothermic oxidation reactions will beselfsustaining even though the temperature of the incoming gases shouldtemporarily fall below the ignition temperature. This hysteresiseffectis due in part to the heat capacity of a catalyst bed and in partto the nature of the specific catalytic composition employed. Most ofthe exothermic heat of reaction is believed to result from the oxidationof carbon monoxide to carbon dioxide, as distinguished from theoxidation of hydrocarbons or oxyhydrocarbons.

Studies of typical urban driving patterns indicate that a largepercentage of driving time is spent at engine conditions of idle andcruising speeds under about 30 miles per hour when the exhaust gastemperature is generally below about 400 F. If the particular catalystemployed under these conditions were to have an ignition temperatureabove 400 F., then obviously little or no conversion of the exhaust gascould be achieved. Furthermore, a considerable portion of auto commutertrafiic consists in short-haul runs beginning with a cold engine; thesensible heat of the exhaust gases during the Warm-up period necessarilyis used to heat up the exhaust manifold, exhaust pipe convertor andcatalyst bed so that a period of from about 5 minutes to an hour ormore, as in a severely cold climate, is required before conversion ofexhaust gases commences. All during such warm-up periods, of course, theexhaust gases pass on through the convertor essentially unchanged andare thence released to the atmosphere.

Waste gas oxidation catalysts are generally constituted in a mannersimilar to naphtha reforming catalysts as well as other hydrocarbonconversion catalysts in that they comprise a high surface arearefractory oxide base or support such as alumina, alumina-silica,aluminazirconia, and the like, upon which is deposited, as byprecipitation or impregnation techniques, one or more activators, e.g.,a catalytically active metal or metal oxide having oxidizing activity.Particularly desirable activators are the metals of the platinum family,especially platinum and palladium. These show excellent conversionactivity for carbon monoxide, hydrocarbons, and oxygenated hydrocarbonsover prolonged periods of time. It has been established that theignition temperature of catalysts in which the activator comprises aplatinum group metal decreases as the weight percentage of platinumgroup metal present, based on the total composite, is increased, up toabout 5% by weight; the relationship is somewhat less than linear,perhaps approximating a hyperbolic function so that at concentrationsabove about 5% by weight, additional amounts of activator do not appearto effect any appreciable reduction of ignition temperature. One way,therefore, to formulate a catalyst having a suitably low ignitiontemperature is to fix the concentration of activator at -acorrespondingly high level, say in the case of platinum, at about 1% byweight, at which level the ignition temperature of the catalyst will bein the vicinity of about 350 F. However this approach is undulyexpensive in commercial practice. Economic studies have shown that inorder to make a pack-aged catalytic convertor complete with catalystcompetitively attractive to the mass motorist market, the platinumcontent of the finished catalyst'should not exceed about 0.2% by weight.Furthermore, on account of the aforesaid hysteresis effect, once thecatalyst bed has been brought up to operating temperature, very goodcarbon monoxide and hydrocarbon conversions can be realized under allconditions of engine operation, e.g. whether at idle, accelerate, cruiseor decelerate, with platinum contents as low as 0.05%0.l% by weightbased on the finished catalyst. It is clear that under normalsteady-state conditions, amounts of platinum substantially in excess ofthis range would be mere surplusage. In fact, extensive tests havedemonstrated the operability of catalysts containing as little as 0.01%platinum. However, catalysts containing below about 0.01% platinum showa marked decrease in stability or ability to sustain high exhaust gasconversions, in the face of contamination by lead and lead compoundswhich are inevitably present in exhaust gases from internal combustionengines operating on gasoline containing tetraethyl lead. In view of theforegoing, the optimum platinum concentration of an exhaust gasoxidation catalyst employing platinum as the activator lies within therange of about 0.05% to about 0.2% by weight; however, the ignitiontemperature of such catalysts is unsatisfactorily high for use withintermittently operated internal combustion engines. A basicshortcoming, then, of platinum catalysts in particular, and of thecatalysts in which the activator comprises other metals in general, isthat those containing amounts of activator low enough to be commerciallyfeasible have relatively high ignition temperatures, while those whichhave satisfactorily lo-w ignition temperatures must contain too muchactivator to be practical.

The present invention is directed to a novel oxidation catalystpossessing remarkably low ignition temperature but containing only avery low percentage by weight of activator. The catalyst is in the formof small particles of a porous high surface area refractory oxide baseor support such as alumina, alumina-silica, alumina-zirconia, etc.,composited with one or more activators such as platinum, cobalt, copper,iron, etc. having oxidizing activity. The essence of the inventionresides in the particular distribution of activator on each individualcatalyst particle. In conventional supported catalysts the activator isdistributed substantially uniformly over the surface of the particle asa regular pattern of extremely small, closely spaced crystals of metalactivator which may exist as the free metal, metal oxide, sulfide,halide or in some chemical or physical complex with the refractorycarrier itself, depending on the particular composition of the catalystand its manner of preparation. In accordance with the present invention,however, the activator is distributed non-uniformly over the surface ofthe particle; [11 other words, it is characterized as having a variabledensity. As used in the specification and claims hereof, the termdensity of activator refers to its surface concentration expressed asweight units per unit area of surface of the support, as for example,micrograms of plati num per square meter of surface area. The surfacearea of the refractory oxide support is defined as that conventionallydetermined by nitrogen BET analysis. The catalyst is prepared in such amanner that the surface of each catalyst particle comprises at leastone, and preferably a plurality of, small localized spots of relativelyhigh activator density, and the remaining surface thereof carries afairly uniform distribution of activator at a substantially lowerdensity. The sum of the areas of such high density regions constitutes aminor portion of the total surface area of the particle, and the totalactivator content of the particle does not exceed a commerciallypractical limit; where the activator is platinum, the total activatorcontent of each particle will be in the range of from about 0.05% toabout 0.2% by weight. A catalyst bed composed of such variable densityplatinum particles operates as follows: at the lowest temperatures, thecombustion starts at those localized sites of each particle having thehigher platinum density. These sites generate enough heat to soon raisethe temperature of the entire particle to the ignition temperature ofits lower platinum density region. Therefore the heating-up mechanismtakes place by way of many small localized high temperature v zoneswhich are distributed throughout the volume of the catalyst bed andwhich come into being more or less simultaneously. Thus from a number ofsmall localized spots of high platinum density, the resulting ignitionchain is quickly propagated throughout the entire catalyst bed, untilthe latter is operative at the higher temperatures necessary forutilization of catalytic areas containing the lower platinum density. Ina preferred form of the invention, the catalyst particles in the bedhave about the same size and shape, and each particle contains more orless the same number of high density spots. This means that, on astatistical average, the high density spots are uniformly dispersedthroughout the entire volume of the bed whereby to promote the mostefficient interparticle transfer of heat by radiation and conduction.

One embodiment of this invention relates to a method of making aparticle-form oxidation catalyst which comprises preparing small solidplastic particles containing a catalytic activator having oxidizingactivity; commingling said plastic particles with a hydrosol of arefractory oxide; gelling the resultant hydrosol-plastic particlemixture and forming hydrogel particles of substantially larger size thanthe plastic particles, each hydrogel particle containing at least oneplastic particle; hydrogel particles at a temperature and for a timesufficient to burn off the plastic to yield refractory oxide particles,the surface of each comprising a localized spot of relatively highactivator density.

In a more specific aspect of the preparation of the catalyst, thehydrogel particles, before calcination thereof, are impregnated with anaqueous solution containing a catalytic metal having oxidizing activityand the impregnated hydrogel particles are than calcined at atemperature and for a time sufficient to burn off the plastic to formrefractory oxide particles, the surface of each comprising one or morelocalized spots of relatively high catalytic metal density and theremaining surface thereof having a uniform distribution of catalyticmetal at a lower density.

Another embodiment of the invention is directed to an oxidation catalystcomprising platinum on a high surface alumina particle having a surfacearea in the range of about to about 220 square meters per gram, a minorportion of the surface area comprising one or more localized spots ofplatinum each at a relatively high density averaging in the range ofabout 10 to about 20,000 micrograms of platinum per square meter ofsurface area, and the remaining surface area thereof having a fairlyuniform distribution of platinum thereon at a relatively low density andcalcining the averaging in the range of about 0.9 to about micrograms ofplatinum per square meter of surface area, the ratio of such highdensity to such low density being at least 3, and the total platinumcontent of the finished particle being in the range of about 0.05% toabout 0.2% by weight.

Another embodiment of this invention concerns a process for burningcombustibles contained in a waste gas stream which comprises contactingthe stream, in admixture with oxygen and at oxidation temperature, witha bed of refractory oxide particles containing one or more catalyticmetals having oxidizing activity, the surface of each particlecomprising one or more localized spots of relatively high catalyticmetal density and the remaining surface thereof having a fairly uniformdistribution of a catalytic metal at lower density.

The use of a catalyst bed composed of catalyst particles having variabledensity activator is quite different from the known concept of employingan active core of ignitor catalyst disposed in a main catalyst bed oflower activity. Arrangements of the latter type contain a cluster orlumped mass of high activity particles designed to function as a zone ofhigh heat capacity and persistent heat retentivity with minimum heatloss therefrom, whereby an active ignition source within the catalyticconvertor is maintained for a considerable time after the engine isturned off. Since a plurality of high activity particles are bunchedtogether, the cold body/hot body contact area ratio is reduced, and ahot particle located centrally within the group can see only other hotparticles rather than cold ones. This means that substantially less heatflow can occur and the heat propagation effect is largely retarded. Thepresent invention, on the other hand, seeks to maximize heat transferfrom each high density site to the remainder of the particle and also tothe surrounding particles by conduction and radiation.

While the preferred catalyst composition of this invention is platinumon alumina, it will be appreciated that the basic principle of variableactivator density is applicable to catalysts of other compositions,including those comprising different bases and/ or activators. Thevarious activators or catalytically active metals which may becomposited with the refractory oxide carrier, in addition to or in lieuof platinum-group metals of the Periodic Table, may comprise, forexample: vanadium, manganese, chromium, molybdenum, tungsten, members ofthe iron group, copper, silver and gold. A particular metal may beemployed individually or in combination with any of the foregoingmetals; however, platinum is desired by reason of its ability to providesustained high activity for 'the oxidation of carbon monoxide,hydrocarbons and oxygenated hydrocarbons. Therefore adesired catalystmay comprise the following: platinum, palladium, other noble metals suchas iridium, ruthenium and rhodium, various mixtures includingplatinum-iron, platinum-cobalt, platinum-nickel, palladium-iron,palladium-cobalt, palladiuIn-nickel, platinum-palladium,palladium-copper-cobalt, platinum copper-lithium-cobalt,platinum-cobalt-copper, copper-cobalt-nickel-platinum,platinum-palladium-cobalt, manganese-platinum,platinum-cobalt-manganese, lithiumplatinum-cobalt,copper-cobalt-lithium. The high density sites may consist of the sameactivator, or mixture of activators, as the uniform low density regionsof each particle. However, it is also within the scope of the inven tionto provide high density sites comprising one activator and low densityregions comprising another activator. For example, the high densityactivator may be platinum, and the low density activator may be iron,and conversely.

In the preparation of the instant catalyst. the activator isincorporated in two steps. Starting with a conventionally formedhydrosol of the desired refractory oxide, there is commingled with thehydrosol small solid plastic particles containing the activator,preferably as a thermally decomposable compound of the activator. Thehydrosolplastic particle mixture is then converted, by conventionalmethods, to hydrosol particles of substantially larger size than theplastic particles, and then gelled, each gel particle containing one ormore plastic particles. The gel particles are dried and impregnated witha solution containing the activator in such concentration as to providea uniform low density distribution of activator over the surface of eachgel particle. The gel particles are then calcined at a temperature andfor a time suflicient to burn off the plastic and decompose theactivator compound contained in the plastic. Removal of the organicinclusion by burning leaves macroholes or channels in the refractoryoxide particle permeable to reactant gases, and the catalytically activemetal remains as a highly localized spot of high density deposited onthe walls of the hole or channel left by removal of organic material.The amount of activator per particle of plastic, the size of the plasticparticles, and the plastic content of the catalyst sol determine thedistribution and local density of the activator in the finishedcatalyst.

The plastic particles may be formed of thermoplastic or thermo-settingresins. A preferred plastic is polyethylene because it is easily shapedinto small particles, its low melting point readily permits the additionof activator thereto in the molten state, and it is easily decomposed atordinary calcination temperatures. Other suitable plastics includepolypropylene, polystyrene, styrene-acrylonitrile, phenol-formaldehyde,urea-formaldehyde, epoxy resins, polyurethanes, vinyl resins such aspolyvinyl chloride and polyvinyl acetate,acrylonitrile-butadiene-styrene polymers, polyamides, andpolyfiuorocarbons such as Teflon and Kel-F.

Other combustible inclusions (for example, carbon black or sulfur) orinert inclusions may be incorporated in the plastic to make its specificgravity approximately equal to that of the hydrosol, so that the plasticwill maintain a uniform dispersion in the hydrosol bulk and dropletsprior to gelation.

While the activator or catalytically active metal may be incorporatedinto the plastic as a powdered free metal, it is preferred that it be athermally decomposable compound of the metal. In the case of the irongroup metals, such thermally decomposable compound may be the nitrate orcarbonate thereof. In the case of platinum, for example, such thermallydecomposable compound may be ammonium chloroplatinate, platinum sulfide,a platinum oxide or a platinum amine complex. The preparation of plasticparticles containing the activator may be accomplished in various ways.One is to encapsulate small crystals of activator with plastic. Anotheris to agitate a slurry of finely divided activator in a molten mass ofthermoplastic and spray or extrude the liquid-solid mixture into aquenching or setting medium. Another is to intimately mix finely dividedactivator with a thermosetting plastic monomer, subject the mixture topolymerization conditions, and then cut or grind the resulting solidmass into fine particles of the desired size. The quantity of activatorper plastic particle will generally range from about 0.1% by weight toabout 10% by weight. Where the activator is platinum, a preferred rangetherefor is 0.1%5% by weight of platinum. The physical shape of theplastic particles may be in the form of granules such as spheres,spheroids, ellipsoids, cylinders, cubes or irregularly shaped granules,or the particles may be elongated threads as where the plastic isdie-extruded. A particularly preferred shape of plastic particles ismicrobeads or microspheres having a diameter in the range of from about10 to about 200 microns.

The refractory oxide base or support is conventionally prepared from ahydrosol thereof. A preferred support comprises a major proportion ofalumina, the term alumina being intended to include porous aluminumoxide in any of its several states of hydration. The support may beessenetially pure alumina or it may be a composite thereof with at leastone other refractory oxide such as silica, zirconia, magnesia, titaniaand the like, and it may also comprise a combined halogen such asfluorine or chlorine. Minor amounts of silica or titania or halogenenhance the cracking activity of the catalyst, zirconia improves itsattrition resistance, and magnesia increases its lead stability in someinstances. Such added oxide or oxides may be present in the finishedcatalyst in an amount within the range of about 0.1% to about 30% byweight, preferably within the range of about 1% to about 10% by weight.Where the support comprises halogen, the halogen may be present in anamount within the range of about 0.1% to about by weight. An aluminahydrosol may be prepared by dissolving in water an aluminum salt such asaluminum chloride, aluminum sulfate, aluminum nitrate, or by digestingaluminum metal in a strong mineral acid such as hydrochloric acid, andthen raising the pH to the desired value by addition of ammonia or otheralkaline medium. Other variables affecting the properties of the sol andof the finished carrier include the aluminumzanion ratio, pH, ionicconcentration, specific aging treatments, and the like, and these may beappropriately controlled by known techniques. When a multi-oxide supportis desired this may be prepared by various suitable methods includingsuccessive precipitation or coprecipitation techniques; for example,silica may be incorporated by adding an alkali metal silicats to analumina sol, or by adding an aluminum salt solution to a freshlyprepared or pre-aged silica sol; zirconium may be incorporated by addinga zirconyl halide to an alumina sol. The activator-containing plasticparticles are then added to and uniformly dispersed in the sol, as byagitation, prior to forrnnig the sol particles and gelation.

Gelation of the sol-plastic particle mixture may be carried out byspraying or injecting the sol into a basic precipitating medium such asammonia or an amine, or by the well-known oil drop method, utilizing aninternal precipitant such as urea or urea-hexarnethylenetetramine,following the procedure set forth in US. Patent 2,620,314. While thefinished catalyst particles may have any physical shape such as spheres,cylinders, pills, or eatrudates, a preferred form of support is thesphere. Alumina or alumina-containing spheres may be readily prepared bythe oil drop method; these spheres may be further given atmospheric orpressure aging treatment, under controlled conditions of pH, temperatureand time, to develop their strength, surface area, pore volume anddensity. The gelled spheres may contain from 1 to about 50 or moreplastic particles, depending on their relative size and activatorcontent of the latter. The preferred diameter range of the finishedcatalyst spheres is about 0.03 to about 0.3 inch, the usual sizes beinginch or /s inch. Diameters below this range may result in excessivecatalyst loss or plugging of the catalyst retaining screens within theconvertor, and diameters above this range may cause channelling,non-uniform space velocity, and poor fluid-solid contact, at least inthe case of the l-pound to -pound catalyst loadings commonly employed invehicular exhaust gas convertors.

After the plastic particle-containing hydrogel particles are formed,they may be dried and impregnated with activator solution, or the dryingstep may be omitted and the undried particles may be directlyimpregnated with activator solution. The drying operation can be carriedout in a batch oven or continuous belt drier at a temperature, forexample, in the range of about 200400 F. The purpose of the impregnatingstep is to provide a low density distribution of activator over thesurface of each catalyst particle. When the activator is platinum, theimpregnating solution may be an aqueous solution of ammoniumchloroplatinate, platinous chloride, platinic chloride,dinitritodiamino-platinum, etc. When the catalyst is to contain othermetallic activators, the impregnating solution may comprise a solublenitrate, sulfate, chlorate, chloride or carbonate of the desiredcatalytically active metal. The activator density can be readilycontrolled by properly adjusting the concentration of the impregnatingsolution.

The impregnated particles are then calcined at a temperature and for atime sufficient to burn off the plastic and decompose the thermallydecomposable activator compound. Generally speaking, calcinationtemperatures of 8001400 F. and exposure times of 10 minutes to 4 hourswill be adequate therefor. When the organic matter is burned off, thecatalytically active metal will be left behind as a highly localizedspot of high density deposited on the walls of the hole or channel leftby removal of the organic material.

The relative amounts of high density activator and loW density activatorare proportioned to provide, as to each catalyst particle, a totalactivator content generally within the range of about 0.03% to about 10%by weight; when the activator is platinum or other platinum group metal,the most effective and economical activator content, as above described,is about 0.05% to about 0.2% by weight. With respect to the relativedensities of activator as between the high density sites and the lowdensity regions, the high activator density should be at least twice thelow density, said densities being expressed in terms of weight units perunit of particle surface area. When platinum is employed as both thehigh density activator and low density activator, the high density ispreferably at least three times the low density; for example, in aplatinum-alumina catalyst prepared according to this invention, theratio of the average high density-to the average low density ispreferably at least 3, and still more preferably is within the range ofabout 3 to about 2,000. A minimum density ratio of 23 is necessary inorder that the heating effect arising from a multiplicity of small hightemperature zones is self-propagating in minimum time.

The physical properties of the instant catalyst, as well as its activityand stability, are dependent to some extent on the specific steps andconditions involved in preparing the refractory oxide support. When thesupport comprises or more by weight of alumina, the finished catalystwill have a surface area of from about to about 220 square meters pergram and an apparent bulk density (ABD) of from about 0.15 to about 0.50gram per cubic centimeter. Higher surface areas of up to about 500square meters per gram may be obtained when increasing proportions ofsilica are incorporated in the support. Representative sphericalplatinum-alumina oxidation catalysts prepared in accordance with thisinvention have the following characteristics:

Sphere diameter, inches 0.30-03 Surface area, mfi/gm. l20-220 ABD,gm./cc. 0.150.50 No. high Pt density sites/ sphere 2-50 High Pt density,D g Pt/m. (av.) 1020,000 Low Pt density, D g Pt/m. (av.) 0.9-10 DH/DL(3.1'.) Total Pt content, each sphere, weight percent 005-02 Thefollowing examples are given to illustrate the present invention and toindicate the benefits aiforded through the use thereof. It is notintended that the invention be limited to the specific reagents,catalyst compositions, concentrations and/or conditions described in theexamples.

EXAMPLE I The ignitiontemperature of a catalyst sample is deter mined asfollows: a 10 cubic centimeter bed of catalyst -is placed in anelectrically heated Vycor tube, through pcrature is reached, the outlettemperature will suddenly begin to rise at a more rapid rate than theinlet temperature until the combustion process lines out, whereupon theinlet and outlet temperatures will again continue to rise, assuming thatadditional heat is still being added to the system, but at equal rates.The point of divergence between inlet and outlet temperatures is thentaken as the ignition temperature.

A conventional spherical platinum-alumina catalyst, designated as A, isprepared by the general method of dissolving aluminum pellets inhydrochloric acid to form an alumina hydrosol. The sol is treated in amanner similar to the procedure set forth in US. Patent No. 2,620,314,involving the mixing of hexamethylenetetramine therewith and droppinginto an oil bath maintained at about 190 F. to form spheres inch indiameter. The spheres are aged in oil and then washed in an aqueoussolution of ammonia, the ammonium hydroxide washed spheres beingsubsequently dried. The spheres are impregnated with platinum by soakingin a dilute solution of ammonium chloroplatinate. Theplatinum-impregnated spheres are then calcined at 1000 F. for 2 hours.This conventional preparation yields catalyst spheres comprising about0.1 platinum by weight uniformly distributed over the surface thereof.

A second spherical platinum-alumina catalyst, desigtrate in an amountand in a manner such that the iron from this source imparts a uniformdistribution of 0.08% by weight of iron to the dried spheres. Thespheres are then calcined at 1000 F. for 2 hours.

Catalyst C and catalysts A and B of Example I are then tested underexhaust gas oxidation conditions. Beds of catalysts A, B and C, disposedin identical catalytic afterburners equipped with air aspirators, areseparately installed in the exhaust line of a passenger automobile andtested under various operating conditions. In one test, the automobileis run on a chassis dynamometer through a standard cycle includingacceleration, low speed cruise, further acceleration, high speed cruise,deceleration and idle, the inlet and outlet gases from the catalyst bedbeing collected and analyzed for hydrocarbon and carbon monoxide contentduring each portion of the cycle, and the overall weighted averageconversions during the entire cycle being calculated. These weightedaverages are given in the table below as those of cyclic operation. Inanother series of tests, the three catalyst beds are separately testedunder steady idling, steady 30 mph. cruise, and steady 60 mph. cruise,the first two providing low inlet temperatures to the catalyst bed andthe third providing high inlet temperatures. In all cases a fuelcontaining 3 milliliters per gallon of tetraethyl lead fluid was used.The results obtained are shown below:

Percent Conversion nated as B, is prepared in accordance with theinvention. Polyethylene microbeads, having a diameter in the range of3080 microns and containing 0.5% by weight of ammonium chloroplatinate,are added to an alumina hydrosol in an amount of 5% by weight. The solis then oil-dropped and aged in the conventional manner to form hydrogelspheres ,4 inch in diameter. The spheres are oven-dried at 300 F. andimpregnated with an aqueous solution of ammonium chloroplatinate in anamount and in a manner such that the platinum from this source imparts auniform distribution of 0.05% platinum by weight to the dried spheres.The spheres are then calcined in air at a temperature of 1000 F. for 2hours. In the course of this step, the polyethylene is burned off andthe ammonium chloroplatinate contained therein is decomposed, leaving anumber of high platinum density sites distributed throughout eachsphere. The total platinum content of each sphere'is the same as that ofcatalyst A, namely 0.1% platinum by weight.

Catalysts A and B are then each subjected to the carbon monoxideoxidation test as described above. Catalyst A has an ignitiontemperature of 415 F. and catalyst B has an ignition temperature of 390F. It is clear that a substantially lower ignition temperature can beobtained with the variable platinum density catalyst than with theconventional catalyst having a completely uniform distribution ofplatinum with respect to each spherical particle, notwithstanding thefact that the total platinum contents are the same in each instance.

EXAMPLE II A spherical platinum-iron-alumina catalyst, designated as C,is prepared in accordance with the invention. Polyethylene microbeads,having a diameter in the range of -80 microns and containing 0.5% byweight of ammonium chloroplatinate, are added to an alumina hydrosol inthe amount of 5% by weight. The sol is then oil-dropped and aged in theconventional manner to form hydrogel spheres inch in diameter. Thesespheres are then dried and impregnated with an aqueous solution offerric ni- It will be seen that while catalysts A, B and C are nearlyequivalent under high temperature conditions, catalysts B and C aremarkedly superior at low temperature conditions of idle and low-speedcruise.

I claim as my invention:

1. Method of making a particle-form oxidation catalyst which comprisespreparing small solid plastic particles containing a catalytic activatorhaving oxidizing activity, said plastic being decomposable atcalcination conditions; commingling said plastic particles with ahydrosol of a refractory oxide; gelling the resultant hydrosol-plasticparticle mixture and forming hydrogel particles of substantially largersize than the plastic particles, each hydrogel particle containing atleast one plastic particle; and calcining the hydrogel particles at atemperature and for a time sufl icient to burn off the plastic to yieldrefractory oxide particles, the surface of each comprising a localizedspot of relatively high activator density.

2. Method of making a particle-form oxidation catalyst which comprisespreparing small solid plastic particles containing a thermallydecomposable compound of a catalytic metal having oxidizing activity,said plastic being decomposable at calcination conditions; comminglingsaid plastic particles with a hydrosol of a refractory oxide; gellingthe resultant hydrosol-plastic particle mixture and forming hydrogelparticles of substantially larger size than the plastic particles, eachhydrogel particle containing at least one plastic particle; andcalcining the hydrogel particles at a temperature and for a timesuificient to burn oil? the plastic and decompose said compound to yieldrefractory oxide particles, the surface of each comprising a localizedspot of relatively high catalytic metal density.

3. The method of claim 2 wherein said metal is platinum.

4. The method of claim 2 wherein said refractory oxide is alumina.

5. Method of making a particle-form oxidation catalyst which comprisespreparing small solid plastic particles containing a thermallydecomposable compound of a catalytic metal having oxidizing activity,said plastic be- 1 1 ing decomposable at calcination conditions;commingling said plastic particles with a hydrosol of a refractoryoxide; gelling the resultant hydrosol-plastic particle mixture andforming hydrogel particles of substantially larger size than the plasticparticles, each hydrogel particle containing at least one plasticparticle; drying the hydrogcl particles; impregnating the dried hydrogelparticles with an aqueous solution containing a catalytic metal havingoxidizing activity; and calcining the impregnated hydrogel particles ata temperature and for a time sufiicient to burn off the plastic anddecompose said compound to yield refractory oxide particles, the surfaceof each comprising a localized spot-of relatively high catalytic metaldensity and the remaining surface thereof having a uniform distributionof catalytic metal at lower density.

6. Method of making a particle-form platinum-alumina catalyst whichcomprises preparing small solid plastic particles containing a thermallydecomposable platinum compound, said plastic being decomposable atcalcination conditions; commingling said plastic particles with analumina hydrosol; gelling the resultant hydrosol-plastic particlemixture and forming alumina hydrogel particles of substantially largersize than the plastic particles, each hydrogel particle containing aplurality of plastic particles; and calcining the hydrogel particles ata temperature and for a time sufficient to burn oif the plastic anddecompose said platinum compound to yield platinum-alumina particles,the surface of each particle comprising a number of localized spots ofrelatively high platinum density.

7. Method of making a particle-form platinum-alumina catalyst whichcomprises preparing small solid plastic particles containing a thermallydecomposable platinum compound, said plastic being decomposable atcalcination conditions; cornrningling said plastic particles with analumina hydrosol; gelling the resultant hydrosol-plastic particlemixture and forming alumina hydrogel particles of substantially largersize than the plastic particles, each hydrogel particle containing aplurality of plastic particles; drying the hydrogel particles;impregnating the dried hydrogel particles with an aqueous solutioncontaining platinum; and calcining the hydrogel particles at atemperature and for a time suificient to burn off the plastic anddecompose said platinum compound to yield platinum-alumina particles,the surface of each particle comprising a number of localized spots ofrelatively high platinum density and the remaining surface thereofhaving a fairly uniform distribution of platinum at lower density.

8. Method of claim 7 wherein said plastic particles are substantiallyspherical and have a diameter in the range of 10200 microns.

9. Method of claim 7 wherein the finished platinumalumina particles aresubstantially spherical and have a diameter in the range of 003-03 inch.

10. Method of claim 7 wherein said plastic is polyethylene.

11. Method of claim 7 wherein said calcination temperature is in therange of 800l400 F.

12. A catalyst prepared by the method of claim 1.

13. A catalyst prepared by the method of claim 7.

References Cited UNITED STATES PATENTS 2,071,119 2/1937 Harger 23-2.22,643,980 6/ 1953 Houdry 252466 2,805,206 9/1957 John et al 2524482,927,902 3/1960 Cramer et al 252466 2,928,792 3/ 1960 Betrolacini252-466 2,929,792 3/1960 Arnold et al 252-448 3,033,800 5/1962 Elliot etal. 252--466 3,254,966 7/1966 Bloch et a1 23-22 3,259,589 6/1966Michalko 252466 FOREIGN PATENTS 686,988 5/1964 Canada.

DANIEL E. WYMAN, Primary Examiner.

P. E. KONOPKA, Assistant Examiner.

